Is a String array subclass of an Object array?

Question

I think I'm missing something basic here. Any explanation or pointers to previously asked questions will be very helpful.

import java.util.Arrays;
import java.util.List;

public class St {

    public static void bla(Object[] gaga) {
            gaga[0] = new Date(); // throws ArrayStoreException
        System.out.println(gaga[0]);
    }

    public static void bla(List<Object> gaga) {
        System.out.println(gaga.get(0));
    }

    public static void main(String[] args) {
            String[] nana = { "bla" };
        bla(nana); // Works fine

        List<String> bla1 = Arrays.asList(args);
        bla(bla1); // Wont compile

            System.out.println(new String[0] instanceof Object[]); // prints true
            System.out.println(nana.getClass().getSuperclass().getSimpleName()); // prints Object
    }

}

So, it seems like a List<String> is not a subclass of a List<Object> but a String[] is a subclass of Object[].

Is this a valid assumption? If so, why? If not, why?

Thanks

Answer 1

Java arrays are covariant, i.e. they allow Object[] foo = new String[2];. But this doesn't mean they are subclasses. String[] is a subclass of Object (although instanceof returns true, String[].class.getSuperclass() returns Object)

Answer 2

Yes, your assumption is valid. As said by @Bozho arrays are covariant, whereas generic collections (such as generic List) are not covariant.

Covariance in arrays is risky:

String[] strings = new String[] { "a", "b" }
Object[] objects = strings;
objects[0] = new Date();  // <-- Runtime error here 
String s = strings[0];
s.substring(5, 3);        // ????!! s is not a String 

The third line fires a runtime exception. If it weren't firing this exception then you could get a String variable, s, that references a value that is not a String (nor a subtype thereof): a Date.